1. Field of the Invention
The present invention broadly relates to the papermaking art and, more specifically, concerns a new and improved forming section or former for a papermaking machine as well as an advantageous method of use thereof.
2. Discussion of the Background and Material Information
Generally speaking, the forming section of the present development for a papermaking machine serves to form a paper web or sheet between a top or upper wire and a lower wire during dewatering of a fiber suspension or fiber stock between the wires. There is employed at both sides of these wires, at the top as well as at the bottom, ledge members or ledges which are arranged along the line of the web forming path and which extend in the cross-machine direction. These ledges are adjustable relative to the wires and contact such wires during operation of the papermaking machine. The ledges are arranged at the upper wire and the lower wire in spaced relationship to one another so that spaces or distances are formed between the neighboring ledges and there are left free gaps for the discharge of the liquid which is to be removed during the dewatering operation.
Forming sections or former of the aforementioned construction can be operated in the direct neighborhood of a headbox. The fiber suspension or stock is sprayed in the form of a flat jet into the intermediate space which is formed by virtue of the convergence of the wires and thereafter such injected fiber suspension is dewatered between the wires. These forming sections can be, however, also provided with a pre-dewatering section or path in which there is ensured that the fiber suspension or stock which effluxes from the headbox is guided upon one of the wires throughout a certain path, and already prior to entry of the fiber suspension into the forming section there is removed a part of the contained water out of the fiber suspension or stock, especially in a downward direction. The pre-dewatering path then follows the forming section of the here described type, and, as a general rule, the pre-dewatered fiber stock or suspension remains reposing upon the wire and through the utilization of a further wire the forming section or former is constructed as a so-called twin wire forming section.
With heretofore known forming sections or formers of the aforementioned type, for instance as known from U.S. Pat. No. 4,769,111, German Patent No. 3,138,133, German Patent No. 3,153,305, German Patent No. 3,546,629 and European Patent No. 0,251,778, a multiplicity of ledges are arranged uniformly distributed over the entire length of the web forming section or path. Through the use of these ledges there is intended to be achieved an improvement in the web formation. It can happen that such distribution of the ledges can lead to damage of the formed paperweb or sheet, particularly at those locations of the web formation section or path which are located behind or upstream of the so-called waterline.
In other constructions of the forming section there is employed from the opposite side a ledge or ledge member which, in each instance, is directed towards the oppositely situated dewatering gap or slot. Such an arrangement is provided at each dewatering gap or slot and there is employed an appreciable pressure, so that the liquid can be expressed out of the paper web or sheet. Under certain circumstances, this pressure can be so great that the paper web which is to be formed is crushed between the wires. In the event of an absence of the pressure, or else if the pressure is too low, then the paper web can be retromoistened from above by the application of the previously expelled liquid, and consequently, this paper web is disadvantageously altered. With the high pressures which are to be applied also the wires are adversely affected and are prematurely worn. Because of the small spacing between the ledges there is a tendency for the intermediate gap between the ledges to become contaminated by, for instance, too much fiber and ash which has been removed from the paper web or sheet.
The forming section or former which is known from German Patent No. 3,546,629 has the lower ledges or ledge members mounted upon a flexible plate. However, with this prior art construction the water which is removed in a downward direction must be laterally discharged between the ledges. As a result, the quantity of removed water is limited and there exists the danger of contamination or soiling of the forming section. Moreover, there cannot be adjusted any individual contact or pressing force of the ledges and the reproducibility of the adjustment is unsatisfactory.
If, as taught for instance in the European Patent No. 0,251,778, a multiplicity of ledges or ledge members located at the lower wire are mounted upon a rigid frame, then there is particularly great the danger of excessively compressing the paper web, damaging the wires and so forth. With this prior art construction of forming section there is also limited the removal of the water and there is simultaneously present an appreciable danger of contaminating the forming section.
With the heretofore known arrangements the provision of the ledges at the lower wire increases the wear of the wire. There is also increased the wire throughput, that is to say, too great a quantity of fines and ashes are washed out of the paper web or sheet. With the heretofore known constructions of forming sections, the web forming pulses, which act upon the ledges, are automatically too high and frequently, as a result thereof, they cause an impairment in the web or sheet formation when working with shear-force sensitive materials.